“So it appeared to me,” replied Thorndyke; “and, as we are in agreement so far, we will carry our investigations a step further.”

He laid the cigar down on the table, and, with the keen, thin-bladed penknife, neatly divided it lengthwise into two halves.

“Ecce signum!” exclaimed Thorndyke, as the two parts fell asunder; and for a few moments we stood silently regarding the dismembered cheroot.  For, about half an inch from the small end, there appeared a little circular patch of white, chalky material which, by the even manner in which it was diffused among the leaf, had evidently been deposited from a solution.

“Our ingenious friend again, I surmise,” said Thorndyke at length, taking up one of the halves and examining the white patch through his lens.  “A thoughtful soul, Jervis, and original too.  I wish his talents could be applied in some other direction.  I shall have to remonstrate with him if he becomes troublesome.”  “It is your duty to society, Thorndyke,” I exclaimed passionately, “to have this infernal, cold-blooded scoundrel arrested instantly.  Such a man is a standing menace to the community.  Do you really know who sent this thing?”

“I can form a pretty shrewd guess, which, however, is not quite the same thing.  But, you see, he has not been quite so clever this time, for he has left one or two traces by which his identity might be ascertained.”

“Indeed!  What traces has he left?”

“Ah! now there is a nice little problem for us to consider.”  He settled himself in his easy chair and proceeded to fill his pipe with the air of a man who is about to discuss a matter of merely general interest.

“Let us consider what information this ingenious person has given us about himself.  In the first place, he evidently has a strong interest in my immediate decease.  Now, why should he feel so urgent a desire for my death?  Can it be a question of property?  Hardly; for I am far from a rich man, and the provisions of my will are known to me alone.  Can it then be a question of private enmity or revenge?  I think not.  To the best of my belief I have no private enemies whatever.  There remains only my vocation as an investigator in the fields of legal and criminal research.  His interest in my death must, therefore, be connected with my professional activities.  Now, I am at present conducting an exhumation which may lead to a charge of murder; but if I were to die to-night the inquiry would be carried out with equal efficiency by Professor Spicer or some other toxicologist.  My death would not affect the prospects of the accused.  And so in one or two other cases that I have in hand; they could be equally well conducted by someone else.  The inference is that our friend is not connected with any of these cases, but that he believes me to possess some exclusive information concerning him—­believes me to be the one person in the world who suspects and can convict him.  Let us assume the existence of such a person—­a person of whose guilt I alone have evidence.  Now this person, being unaware that I have communicated my knowledge to a third party, would reasonably suppose that by making away with me he had put himself in a position of security.